1. Field of the Invention
This invention relates to the preparation of powdered polymers and is more particularly concerned with the production of powdered polymers having particles of substantially uniform size and shape.
2. Description of the Prior Art
Numerous methods of preparing polymers in powder form, either in the isolated state or as a dispersion in a liquid medium, are described in the art. These methods include direct precipitation by treating a solution of the polymer with a solvent in which the polymer is insoluble, spray drying the polymer from a solution thereof, or simply grinding the polymer, optionally dispersed in a liquid medium. These various methods have been applied to polymers which have already been formed and isolated in the solid state and have also been adapted to be used in conjunction with the process for preparation of the polymers. Illustrative of the latter type of process are those shown in U.S. Pat. Nos. 3,420,796; 3,894,994; and 3,917,741. Illustrative of the methods employed in reducing already formed polymers to powder form are U.S. Pat. No. 3,185,660, which shows dissolving polymers such as polyolefins and polyamides in appropriate solvents by heating under pressure and flashing the hot solution into a second vessel, and U.S. Pat. No. 3,586,654, which shows heating polymers such as polyethylene above the melting point in the presence of a non-solvent liquid and a surfactant and allowing the mixture to cool. Unfortunately, none of the above methods yield powders having reproducibly uniform particle size.
U.S. Pat. No. 3,772,252 shows the preparation of polyester polymers in powder form by dissolving the polymer in a water-immiscible solvent, emulsifying the resulting solution in water, and precipitating the polymer in powder form from the emulsion by adding a solvent in which the polymer is insoluble. U.S. Pat. No. 3,847,886 shows the same process applied to the conversion of water-insoluble vinyl polymers to powder form. The processes described in these latter references are potentially attractive for commercial use in producing powders having particles of spherical shape and relatively uniform size, but they cannot be applied to polymers which do not yield emulsions in aqueous media.
It is an object of this invention to convert solid polymers to powder form wherein the particles are of substantially uniform size and shape. It is a further object of the invention to provide a process for accomplishing this result which is relatively simple and does not require the consumption of large quantities of energy or the use of complex equipment. It is yet a further object of the invention to provide a process which is of application to a relatively broad class of polymers including polymers having relatively high resistance to heat degradation.